When comparing ClamXav vs Little Snitch, the Slant community recommends Little Snitch for most people. In the question“What are the best Mac OSX apps for someone that's new to Apple?” Little Snitch is ranked 7th while ClamXav is ranked 28th. The most important reason people chose Little Snitch is:
Little Snitch rules work even at login screen. Even when receiving attempted connection alerts, once logged in LS has stopped and requested permission for them.
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Pros
Pro Monitor directories easily
The Sentry Feature monitors specific directories 24/7 as well as removable media which includes USB & external drives, all those .dmg files you open when installing apps etc. I add all directories that files are downloaded as a "screening" process.
Pro Lean
Not overly invasive or processor hungry as other antivirus programs
Pro Protected even at login screen
Little Snitch rules work even at login screen. Even when receiving attempted connection alerts, once logged in LS has stopped and requested permission for them.
Pro Comparatively easy to use
Easier to use than traditional firewall software, but more powerful due to design and workflow.
Pro Protects your privacy
Protects your privacy and helps to prevent unwanted connections.